Adsorption of CO_2, CH_4, N_2O, and N_2 on MOF-5, MOF-177, and Zeolite 5A

摘要

Adsorption equilibrium and kinetics of CO_2, CH_4, N_2O, and N_2 on two newly discovered adsorbents, metal-organic frameworks MOF-5 and MOF-177 and one traditional adsorbent zeolite SA were determined to assess their efficacy for CO_2, CH_4. and N_2O removal from air and separation of CO_2 from CH_4 in pressure swing adsorption processes. Adsorption equilibrium and kinetics data for CO_2, CH_4, N_2O, and N_2 on all three adsorbents were measured volumetrically at 298K and gas pressures up to 800 Torr. Adsorption equilibrium capacities of CO_2 and CH_4 on all three adsorbents were determined gravimetrically at 298 K and elevated pressures (14 barfor CO_2 and 100 bar for CH,). The Henry's law and Langmuir adsorption equilibrium models were applied to correlate the adsorption isotherms, and a classical micropore diffusion model was used to analyze the adsorption kinetic data. The adsorption equilibrium selectivity was calculated from the ratio of Henry's constants, and the adsorbent selection parameter for pressure swing adsorption processes were determined by combining the equilibrium selectivity and working capacity ratio. Based on the selectivity and adsorbent selection parameter results, zeolite 5A is a better adsorbent for removing CO_2 and N_2O from air and separation of CO_2 from CH_4, whereas MOF-177 is the adsorbent of choice for removing CH, from air. However, both MOF adsorbents have larger adsorption capacities for CO_2 and CH_4 than zeolite 5A at elevated pressures, suggesting MOF-5 and MOF-177 are better adsorbents for CO_2 and CH_4 storage. The CH_4 adsorption capacity of 22 wt.% on MOF-177 at 298K and 100 bar is probably the largest adsorption uptake of CH_4 on any dry adsorbents. The average diffusivrty of CO_2, CH_4 and N_2O in MOF-5 and MOF-177 is in the order of 10~(-9) m~2/s, as compared to 10~(-11)m~2/s for CO_2, CH_4 and N_2O in zeolite 5A. The effects of gas pressure on diffusivity for different adsorabte-adsorbent systems were also investigated.